1. Field of the Invention
The present invention relates to an image forming apparatus for copying machines and printers using an electrophotographic process, and more specifically relates to an image forming apparatus constructed so as to bring a spacing roller attached to a developing material supplying means at a predetermined positional relationship into contact with an image bearing member to maintain a uniform spacing between the image bearing member and the developing material supplying member and supply developing material from the developing material supplying means to the image bearing means.
2. Description of the Prior Art
Conventional image forming apparatus using electrophotographic processes form an electrostatic latent image on the surface of an image bearing member by electrically charging the image bearing member with a charging means and irradiating the charged region of the image bearing member with light corresponding to a document image. The thus formed electrostatic latent image on the surface of the image bearing member is then developed by a developing device that supplies developing material thereto, and the developed image is transferred to a transfer sheet and fixed thereon.
The previously mentioned developing device may be any of several well known types of developing devices for developing electrostatic latent images such as, for example, a magnetic brush developing device using either a two-component developing material comprising a toner and a magnetic carrier or a monocomponent developing material comprising a magnetic toner. A magnetic brush developing device maintains a magnetic brush formed of developing material on the surface of a developing material carrying member and transports the developing material from the developing material carrying member to the developing region to develop an electrostatic latent image. Another type of common developing device applies a bias voltage to induce the developing material to "jump" from the developing material carrying member to the electrostatic latent image.
The aforesaid types of developing devices must provide a spacing between the developing material carrying member and the image bearing member, i.e., provide a developing gap, so as to assure suitable developing.
Means for setting the aforesaid developing gap are disclosed, for example, in U.S. Pat. No. 3,987,756 and Japanese Patent application Ser. No. 1-62562, which provide a spacer collar at the end of the rotatable shaft of a developing roller used as the developing material carrying member. The aforesaid spacer collar sets a developing gap by abutting the surface on the edge of a photosensitive drum which is used as the electrostatic latent image bearing member.
In the aforesaid type of developing gap setting means, however, the spacer collar is normally exposed to the airborne toner particles floating near the surface of the photosensitive drum and elsewhere within the image forming apparatus. These airborne toner particles become interposed between the surface of the collar and the surface of the photosensitive drum and gradually adhere to and accumulate on said surfaces. The aforesaid toner deposits reduce the precision of the developing gap and result in irregular image density and inferior image quality.
The previously mentioned Japanese Patent application Ser. No. 1-62562 proposes the use of a cleaning member to wipe the surface of the photosensitive drum on the developing roller side away from the spacer collar, but the use of such a cleaning member does not prevent the adhesion of the airborne toner particles on the spacer collar.
A related problem arises from damage to members that come into contact with the spacer collar. For example, when the spacer collar comes into contact with the edge of the photosensitive drum, the photosensitive layer of the drum is gradually peeled away and results in gradually reducing the precision of the developing gap, while also allowing the particles of the photosensitive layer peeled from the drum to become mixed with the developing material, thereby contaminating the developing material with a foreign substance.
A means for eliminating the previously described disadvantages has been proposed wherein the photosensitive layer is previously peeled from the edge portion of the photosensitive drum that abuts the spacer collar so that the spacer collar makes contact with the edge portion of the photosensitive member from which the photosensitive layer has already been peeled.
However, the operation by which the photosensitive layer is peeled from the edge portion of the photosensitive drum is very problematic, detracts from the producibility of the photosensitive drum, and increases production costs. Further, the electrical potential of the edge portion of the photosensitive drum which has had the photosensitive layer peeled therefrom does not rise even when the peeled photosensitive drum is electrically charged by the charging means. Therefore, when a reversing development process is used in a printer or the like, a quantity of the developing material adheres to the edge portion of the photosensitive drum that abuts the spacer collar. The aforesaid adhered developing material becomes interposed between the edge portion of the photosensitive drum and the spacer collar, thereby leading to further disadvantages such as reducing the precision of the developing gap and causing loss of stability in producing high quality images.